README.rst

JMESPath

JMESPath (pronounced "james path") allows you to declaratively specify how to
extract elements from a JSON document.

For example, given this document:

{"foo": {"bar": "baz"}}

The jmespath expression foo.bar will return "baz".

JMESPath also supports:

Referencing elements in a list. Given the data:

{"foo": {"bar": ["one", "two"]}}

The expression: foo.bar[0] will return "one".
You can also reference all the items in a list using the *
syntax:

{"foo": {"bar": [{"name": "one"}, {"name": "two"}]}}

The expression: foo.bar[*].name will return ["one", "two"].
Negative indexing is also supported (-1 refers to the last element
in the list). Given the data above, the expression
foo.bar[-1].name will return "two".

The * can also be used for hash types:

{"foo": {"bar": {"name": "one"}, "baz": {"name": "two"}}}

The expression: foo.*.name will return ["one", "two"].

API

The jmespath.py library has two functions
that operate on python data structures. You can use search
and give it the jmespath expression and the data:

This is useful if you're going to use the same jmespath expression to
search multiple documents. This avoids having to reparse the
JMESPath expression each time you search a new document.

Options

You can provide an instance of jmespath.Options to control how
a JMESPath expression is evaluated. The most common scenario for
using an Options instance is if you want to have ordered output
of your dict keys. To do this you can use either of these options:

Custom Functions

The JMESPath language has numerous
built-in functions, but it is
also possible to add your own custom functions. Keep in mind that
custom function support in jmespath.py is experimental and the API may
change based on feedback.

If you have a custom function that you've found useful, consider submitting
it to jmespath.site and propose that it be added to the JMESPath language.
You can submit proposals
here.

To create custom functions:

Create a subclass of jmespath.functions.Functions.

Create a method with the name _func_<your function name>.

Apply the jmespath.functions.signature decorator that indicates
the expected types of the function arguments.

Provide an instance of your subclass in a jmespath.Options object.

Below are a few examples:

import jmespath
from jmespath import functions
# 1. Create a subclass of functions.Functions.# The function.Functions base class has logic# that introspects all of its methods and automatically# registers your custom functions in its function table.classCustomFunctions(functions.Functions):
# 2 and 3. Create a function that starts with _func_# and decorate it with @signature which indicates its# expected types.# In this example, we're creating a jmespath function# called "unique_letters" that accepts a single argument# with an expected type "string".@functions.signature({'types': ['string']})
def_func_unique_letters(self, s):
# Given a string s, return a sorted# string of unique letters: 'ccbbadd' -> 'abcd'return''.join(sorted(set(s)))
# Here's another example. This is creating# a jmespath function called "my_add" that expects# two arguments, both of which should be of type number.@functions.signature({'types': ['number']}, {'types': ['number']})
def_func_my_add(self, x, y):
return x + y
# 4. Provide an instance of your subclass in a Options object.
options = jmespath.Options(custom_functions=CustomFunctions())
# Provide this value to jmespath.search:# This will print 3print(
jmespath.search(
'my_add(`1`, `2`)', {}, options=options)
)
# This will print "abcd"print(
jmespath.search(
'foo.bar | unique_letters(@)',
{'foo': {'bar': 'ccbbadd'}},
options=options)
)

Again, if you come up with useful functions that you think make
sense in the JMESPath language (and make sense to implement in all
JMESPath libraries, not just python), please let us know at
jmespath.site.

Testing

In addition to the unit tests for the jmespath modules,
there is a tests/compliance directory that contains
.json files with test cases. This allows other implementations
to verify they are producing the correct output. Each json
file is grouped by feature.

Discuss

Join us on our Gitter channel
if you want to chat or if you have any questions.